This invention relates to valves and more particularly to a non-invasive system and method for the inspection of valves.
It may be explained here that generally a check-valve includes a housing and a movable element mounted in the housing for movement between an open position and a closed position and intermediate positions between the open and closed positions. The check-valve operates by allowing flow in one direction when the movable element is in the open position while preventing flow in the other when the movable element is in the closed position. The check valve has no external moving parts and, therefore, the position of the movable element and its integrity cannot be evaluated with normal visual inspection methods without valve disassembly.
Failures of a few check valves in applications directly related to a safe shut down of a nuclear powered electrical generating unit during nuclear power plant operations led to a review of all check valve maintenance actions and failures. INPO (Institute of Nuclear Power Operations) published the results of the review in a Significant Operating Experience Report (SOER) No. 86-03 entitled "Check Valve Failures or Degradation" in October, 1986. The conclusions of this report were that the major causes of check valve failures were primarily due to misapplication and inadequate preventative maintenance.
As a result of INPO's SOER 86-03, the electric industry worked with EPRI (the Electric Power Research Institute) and formed a program to address the needs of the industry. In 1988, EPRI issued a report entitled "Application Guidelines for Check Valves in Nuclear Power Plants." The EPRI report provided guidelines recommending the use of non-invasive inspection techniques to verify proper operation of check valves.
Currently, inspection of check valves is generally accomplished by the disassembly of the valve and visually inspecting the internals. There is a very limited use of non-invasive inspection techniques consisting of ultrasonic, acoustic or, to a limited extent, magnetic techniques. Acoustic techniques involve the detection of structural-borne noise, i.e., acoustic energy or vibrations, emanating from the internal workings of the valve. The acoustic technique generally employs a piezoelectric crystal sensor, such as an accelerometer, mounted on the valve housing. All structural-borne acoustic energy waves or vibrations are detected by the acoustic sensor and converted by it to electric analog voltage signals or data representative of acoustic energy. The data is recorded and then analyzed in an attempt to diagnose which internal valve condition the data is indicating.
Although the indications of various conditions and/or "problems" within the valve can be detected as vibrations by the acoustic sensor, the interpretation of the data as to which problem the acoustic energy or vibrations is indicating is difficult. For example, it is difficult at times, to differentiate between vibrations caused by impacts between worn parts and impacts which are expected in normal operation of the valve, i.e., impacts which occur upon the opening and closing of the valve. Also vibrations created by impacts caused by worn parts as the movable element fluctuates between open and closed positions can be misinterpreted as vibrations created by impacts caused by the movable member striking the valve housing or a valve stop in its fully opened position. It is also possible for the entire movable member to be missing with just the mounting arm which mounts the movable member to the housing remaining and obtain vibrations and resulting acoustic signatures in the data that might be misinterpreted as vibrations of impacts caused by the movable member striking the valve stop when, in fact, it is the mounting arm which is striking some portion of the valve's internal structure.
Magnetic techniques for the inspection of valves currently relate to systems which provide information regarding the position of the movable element of the valve. The magnetic technique involves the use of a permanent magnet mounted on the movable element to provide a varying magnetic field as the position of the movable element changes. A magnet field sensor is used to measure the magnetic field strength from a point outside the valve. As magnetic field strength changes, the sensor will indicate the position of the movable element. Knowing the position of the movable element permits limited diagnostic evaluation of check valves. For example, a fluctuating movable element will be evident using the magnetic technique. Proper seating of the movable element upon closure, however, would not be evident using magnetic techniques although the position of the movable element would indicate closed. Also, and more importantly, worn internal parts would not be evident using magnetic techniques.
Because of these and other difficulties associated with check valve inspection techniques presently employed, there now exists a need and a strong demand for an economical, viable means for the non-invasive inspection of check valves to verify proper operation without the disadvantages of the current inspection techniques.